Apparatus for redirecting radiation

ABSTRACT

An apparatus for redirecting radiation including a polarization dependent reflector and an optical component able to transform the polarisation of radiation as it passes through the component. Radiation having a first polarisation component impinging upon the reflector from a source is reflected towards a scene area. Radiation not having this polarisation component passes through the reflector. The radiation having the first polarisation is then transmitted towards a scene via the optical component. As the radiation passes via the optical component, reflects back from the scene and passes again via the optical component, the polarisation is changed to state where it can now pass through the polarisation dependent reflector. The apparatus allows a scene to be illuminated as if the illuminator were positioned at the viewpoint of an imaging system, and so aids uniform illumination of the scene. The apparatus is particularly suited to mm-wave systems, wherein the reflector may comprise a parallel wire grid polarizer, and the optical component may comprise a quarter-wave plate.

This invention relates to an apparatus for redirecting radiationparticularly suitable for electromagnetic imaging systems. Inparticular, the invention relates to a means for illuminating a subjectfrom the direction of a receiver or imager, without obscuring thesubject from the imager or receiver. The invention has particularutility at millimetre wavebands, but may also be used at otherwavelengths.

When imaging a scene using a millimetre wave imaging system it iscommon, particularly when operated indoors, to use some sort ofillumination source in combination with the imager to increase thecontrast of the scene, and hence improve image quality. Ideally, thisillumination would be delivered uniformly from all angles. In practicethis has proven to be difficult, so systems often compromise by havingbanks of illuminators above, below, and at each side of the scene, thatcrudely approximate to a uniform illumination. One important problemthat occurs with this method is that the scene cannot be illuminatedfrom the viewpoint of the receiver—which is the position where anilluminator is most beneficial in terms of providing a satisfactorilyilluminated scene. The illuminators themselves are typically quitelarge, as they need to ideally provide uniform illumination coverage,but are constrained by the limitations of the wavelength at which theywork, so clearly, any attempt to place an illumination source at theposition of the receiver would block out the view of the scene from thereceiver itself.

Positioning a mirror to reflect the radiation towards the target alongthe boresight of the imager would also be unsatisfactory, as then themirror would be in the path of the radiation returning from the targetto the imager. A partially reflecting mirror could be used, but is notideal as the radiation would be attenuated each time it was incidentupon the mirror.

Patent application PCT/GB98100985 contains details of a scanningapparatus of the type that may be used along with the current invention.This application discloses apparatus for providing to the receiversradiation that has been scanned from across the desired field of view bymeans of a rotating reflector element, and also features a foldedoptical path that incorporates the rotating reflector, making the systemmore compact. It does not however address the problem of satisfactoryillumination of a target area before the radiation reaches this scanningmechanism.

According to the present invention there is provided an apparatus forredirecting radiation, wherein the apparatus is able to receiveradiation along a first axis or plane and redirect its passage along asecond axis or plane, and the apparatus comprises:

-   -   a polarisation dependent reflector, arranged to reflect        radiation having a first polarisation state and pass radiation        having a second polarisation state; and    -   an optical component for altering the polarisation state of        radiation passing therethrough adapted such that radiation in        the first polarisation state passing through the optical        component towards a scene, reflected therefrom and passing back        through the optical component is transformed to the second        polarisation state.

The polarisation dependent reflector may be planar, or may be curved soas to direct the radiation from an illuminator as desired to selectedparts of the target. The polarisation dependent reflector will largelyreflect components of radiation having a first polarisation, but willlargely transmit components not having the first polarisation. The firstpolarisation is preferably a linear polarisation. Any radiationtransmitted through the polarisation dependent reflector at this stageneed not play any further role in illuminating the target.

The optical component through which the radiation passes from thereflector to the target preferably comprises a quarter wave plate. Thiswill have the effect of transforming linearly polarised radiationincident upon it to circularly polarised radiation, either right handed(RH) or left handed (LH), according to the properties of the quarterwave plate and the reflector. Radiation that reflects from the targetwill have a “handedness” opposite to the radiation incident upon thetarget, so RH polarisation will go to LH and vice versa.

Some of the radiation reflected from the target will be directed towardsthe imager. When this radiation passes through the quarter wave platefor the second time it will modify the polarisation of the radiationsuch that it is linear and orthogonal to the radiation originallyreflected by the reflector towards the target. Radiation having thismodified state of polarisation will thus pass unimpeded through thepolarisation dependent reflector plate on towards the imager.

Some imagers are only sensitive to one particular polarisation. In thiscase it is important to arrange the polarisation dependent reflector inthe correct manner such that it passes radiation from the target that isof the same polarisation state as required by the imager.

For systems operating at millimetre wave frequencies, the quarter waveplate is preferably a meanderline. Meanderlines are known in the priorart. See, for example, U.S. Pat. No. 3,754,271. Systems operating atdifferent frequencies may use alternative technologies when implementingthe quarter wave plate.

The illumination source may be a solid state noise source. The output ofthe noise source may be amplified using standard means to produce thedesired illumination levels. The noise source may be distributed, suchthat it comprises a plurality of separate noise sources in differentlocations. This may provide a more even illumination of the target.

Note that the term “optical” as used in this specification does notlimit the application to visible optical wavelengths. The term merelyimplies that that to which it refers may be analysed using optical orquasi optical techniques. Further, note that an “optical component” asmentioned in this specification may comprise a simple optical component,or may comprise a compound component formed from a plurality of moresimple optical components.

According to another aspect of the invention there is provided anillumination source arranged to redirect radiation along the boresightaxis of an imaging system, comprising:

-   -   a source of radiation at a frequency compatible with the imaging        system;    -   a polarisation dependent reflector, adapted to receive-radiation        from the illumination source, and arranged to reflect radiation        having a first polarisation state and pass radiation having a        second polarisation state; and    -   an optical component for altering the polarisation state of        radiation passing therethrough adapted such that radiation in        the first polarisation state passing through the optical        component towards a scene, reflected therefrom and passing back        through the optical component is transformed to the second        polarisation state.

The apparatus of the present invention may be integrated into a chamber,which may also contain additional sources of illumination and an imagingsystem.

According to a further aspect of the invention there is provided amethod of changing the apparent source direction of radiation comprisingthe steps of:

-   a) receiving radiation from an illumination source and reflecting    only radiation having a first polarisation state and passing    radiation having a second polarisation state, by means of a    polarisation dependent reflector;-   b) processing the reflected radiation using an optical component,    whereby the optical component changes the polarisation state of the    radiation, and passing this processed radiation towards a target    area;-   c) passing at least some of the radiation reflected from the target    area back towards the optical component; and-   d) changing the polarisation of the radiation reflected from the    target area to the second polarisation state such that it can pass    through the polarisation dependent reflector.

According to a yet further aspect of the invention there is provided amethod of illuminating a target area using an off-axis illuminationsource comprising the steps of:

-   a) providing an illuminator generating radiation at a desired    wavelength-   b) reflecting radiation from the illuminator having a first    polarisation state and passing radiation having a second    polarisation state, by means of a polarisation dependent reflector;-   c) processing the reflected radiation using an optical component,    whereby the optical component changes the polarisation state of the    radiation, and passing this processed radiation towards a target    area;-   d) passing at least some of the radiation reflected from the target    area back towards the optical component; and-   e) changing the polarisation of the radiation reflected from the    target area to the second polarisation state such that it can pass    through the polarisation dependent reflector.

The invention will now be described in more detail, by way of exampleonly, with reference to the following drawings, in which:

FIG. 1 diagrammatically illustrates a prior art chamber notincorporating the current invention;

FIG. 2 diagrammatically illustrates an embodiment of the currentinvention incorporated into a chamber having an imager and illuminationsources;

FIG. 3 diagrammatically illustrates the polarisation states of theradiation as it passes through various stages of one embodiment of theinvention; and

FIG. 4 diagrammatically illustrates the detail of a meanderline used totransform polarisation states of millimetre wave energy.

FIG. 5 diagrammatically illustrates the track pattern on a meanderlineused in an embodiment of the present invention.

FIG. 6 diagrammatically illustrates a second embodiment of the currentinvention.

Shown in FIG. 1 is a plan view of an imaging chamber 1, as typicallyused when employing a millimetre wave imager 3 indoors to view a target2. Arrays of illuminators 4 are positioned along the sides (andsometimes the top and floor also) of the chamber, to generate a contrastbetween the target 2 and the background 5 as seen by the imager 3.

The ideal chamber provides uniform illumination of the target 2 from alldirections. This is particularly important in millimetre wave systems,due to the relatively long wavelength of the radiation when compared tovisible frequencies leading to much more specular reflection of theradiation. This can lead to dark spots if the target happens to bereflecting an area that is not providing illumination. Illuminators 8are shown that provide side-on illumination, and partial illuminationfrom the front of the chamber. However, millimetre wave imagers may bequite large in frontal area, and so can take up a significant portion ofthe area 6 at the front of the chamber 1. This area 6 then not availableto use as a source of illumination of the target 2.

Any elements of the target that are so positioned that they wouldotherwise reflect radiation from the area 6 back to the area 6 are hencenot able to do so, and therefore these elements may not be optimallyimaged by the imager 3. Surfaces that are parallel or almost parallel tothe imager viewplane are particularly susceptible to this problem.

Shown in FIG. 2 is a chamber that incorporates an illumination reflector9 according to the present invention. It will be seen that there areilluminators 4 positioned as in a prior art chamber, but there is afurther illuminator 8 positioned towards the front of the chamber 1. Bymeans of the invention, this illuminator 8 is able to illuminate thetarget as if the illuminator 8 were cosited with the imager 3, i.e. asif the illuminator 8 and the imager 3 occupied the same spatialposition. This is because the illumination reflector 9 redirects theradiation from the illuminator 8 along the boresight of the imagertowards the target, but provides minimal interference with the reflectedillumination from the target back towards the imager 3.

Best shown in FIG. 3 is the detail of the illumination reflector 9 andhow it is able to reflect radiation along the boresight of the imager 3,by modifying the polarisation of the illumination radiation, withoutblocking the target 2 (see FIG. 2) from the imager 3. An illuminationsource 8 provides unpolarised radiation, shown by the arrow A, which isdirected generally towards the polarisation dependent reflector 10. Thisreflects all vertical polarisation components of the radiation, whilstallowing horizontal polarisation components to pass straight through.The vertical components are reflected towards the target, as shown byarrow B, by means of arranging the angle of the polarisation dependentreflector 10 relative to the imager 3 and target area 2 in a suitablefashion. Horizontally polarised components pass through the polarisationdependent reflector 10 and play no significant part in the illuminationof the target. Before the vertically polarised radiation hits the targetit passes through a quarter wave plate 11, which converts the verticalpolarisation to RH circular polarisation, shown by the arrow C. As aresult of the physical process of reflections of circular polarisation,the “handedness” of the polarisation swaps when the radiation reflectsfrom an object. Therefore, reflections of this radiation from thetarget, shown by arrow D, get converted to LH circular polarisation.

The LH reflections that are directed back towards the quarter wave plate11 have their polarisation state changed to horizontal polarisation bythe action of the quarter wave plate. This polarisation, shown by arrowE, is able to pass unimpeded through the polarisation dependentreflector 10 to the front end optics of the imager 3. The imager 3 inthis case is sensitive only to horizontal polarisation.

The polarisation dependent reflector plate 10 comprises of a grid ofparallel conducting tracks printed onto a non conducting substrate.Components of the electric field that are in line with the grid axiswill be reflected, whist components perpendicular to the grid will passstraight through. The width of each track of the grid is 180 μm, whilstthe track pitch is 40 μm. Such a grid is suitable for operation at Kaband, but the grid should be adapted as appropriate for operation withindifferent frequency bands. Design rules for producing a wire gridpolariser for a particular operating frequency are well known in theart, and further details will not be provided herein.

The optical component that acts as a quarter wave plate used in thecurrent embodiment comprises a meanderline polarisation twister. FIG. 4shows the detail of the meanderline to be used in a Ka band embodimentof the current invention. Four substrates, each having a series ofcopper tracks arranged in a square-wave formation are sandwichedtogether, along with blank spacer boards that maintain the correctdistance between each of the active layers. The two outer active layers12 (Grid type A) are 20 thousandths of an inch (thou) thick, as are theinner two active layers 13 (Grid type B). The central spacer board 15 is15 thou thick, whereas the other spacers 14 are 20 thou thick. Thematerial used for the substrates and spacer boards is Arlon AD270,available from Holders Technology UK Ltd, Tweedbank Industrial Estate,Galashiels, Scotland, TD1 3RS.

FIG. 5, along with Table 1 show the detail of the tracks that make upeach of the panels of the meanderline, with the detailed dimensions ofthe various elements of the tracks in table 1, where w1 and w2 arelinewidths, b is the periodicity, h is the height, and a is the pitch.TABLE 1 Track dimensions, millimetres Grid type a b h w1 w2 A 0.79 3.751.02 0.16 0.11 B 1.17 3.75 1.59 0.29 0.38

Necessary modifications to the design of the meanderline to account fordifferent operating frequencies will be known to those skilled in therelevant arts, and will not be discussed further herein. Further detailsrelating to meanderlines may be found in the following references: L.Young et al., IEEE Transactions on Antennas and Propagation, vol AP21,pp 376-378, May 1973, and R A Chu et al, IEEE Transactions on Antennasand Propagation, vol AP35, pp 652-661, June 1987. Details of some otherdevices that may be used in place of a meanderline for the opticalcomponent 11 are provided in The International Journal of Infrared andMillimeter Waves, Vol 2, No 3, 1981.

FIG. 6 shows a second embodiment of the current invention, wherein thepolarisation dependent reflector 10 has been divided into twoindependent sections 10 a, 10 b, each arranged to reflect towards thetarget radiation coming from two different directions. This arrangementmeans that the length of the reflector 10 along the axis of the imagerboresight is half as much as with the embodiment shown in FIG. 2, thusleading to a more compact illumination arrangement. Of course, however,two illuminators 8 are needed here to provide similar coverage to theprevious embodiment. However, each one need only be half the size andhalf the power.

A third embodiment has the polarisation dependent reflector 10 dividedinto four sections that reflect radiation towards the target from fourdifferent directions. These four sections together form a pyramid shapethat has been oriented such that the “top” of the pyramid now pointsbroadly towards the target area 2. In this case, four illuminators 8 arerequired to ensure maximum coverage of the target area 2—one on thebase, one overhead, and one at each side.

The skilled person will be aware that other embodiments within the scopeof the invention may be envisaged, and thus the invention should not belimited to the embodiments as herein described.

1. An apparatus for redirecting radiation at millimetre wavelengths orthereabouts for the illumination of a target for an imager, wherein theapparatus is able to receive radiation along a first axis or plane andredirect its passage along a second axis or plane, and the apparatuscomprises: a polarisation dependent reflector, arranged to reflectradiation having a first polarisation state and pass radiation having asecond polarisation state; and an optical component for altering thepolarisation state of radiation passing therethrough adapted such thatradiation in the first polarisation state passing through the opticalcomponent towards a scene, reflected therefrom and passing back throughthe optical component is transformed to the second polarisation state.2. An apparatus for redirecting radiation as claimed in claim 1 whereinthe optical component comprises a quarter wave plate.
 3. An apparatusfor redirecting radiation as claimed in claim 2 wherein a meanderline isused as the quarter wave plate.
 4. An apparatus for redirectingradiation as claimed in claim 1 wherein the polarisation dependentreflector comprises a plurality of parallel conducting tracks.
 5. Anapparatus for redirecting radiation as claimed in claim 1 wherein thepolarisation dependent reflector is arranged to reflect radiation comingfrom a single general direction.
 6. An apparatus for redirectingradiation as claimed in claim 1 wherein the polarisation dependentreflector is divided into two or more parts, each arranged to reflectradiation coming from a different general direction towards the scene.7. (canceled)
 8. An illumination source arranged to redirect millimetrewave radiation at millimetre wavelengths or thereabouts along theboresight axis of a imaging system sensitive to said wavelengths,comprising: a source of radiation at a frequency compatible with theimaging system; a polarisation dependent reflector, adapted to receiveradiation from the illumination source, and arranged to reflectradiation having a first polarisation state and pass radiation having asecond polarisation state; and an optical component for altering thepolarisation state of radiation passing therethrough adapted such thatradiation in the first polarisation state passing through the opticalcomponent towards a scene, reflected therefrom and passing back throughthe optical component is transformed to the second polarisation state.9. A chamber for viewing a target at millimetre wave frequencies,wherein incorporated into the chamber is an apparatus for redirectingradiation as claimed in claim
 1. 10. A method of changing the apparentsource direction of radiation of millimetre wavelengths, or thereabouts,comprising the steps of: a) receiving said radiation from anillumination source and reflecting only radiation having a firstpolarisation state and passing radiation having a second polarisationstate, by means of a polarisation dependent reflector; b) processing thereflected radiation using an optical component, whereby the opticalcomponent changes the polarisation state of the radiation, and passingthis processed radiation towards a target area; c) passing at least someof the radiation reflected from the target area back towards the opticalcomponent; and d) changing the polarisation of the radiation reflectedfrom the target area to the second polarisation state such that it canpass through the polarisation dependent reflector.
 11. A method asclaimed in claim 10 wherein the optical component comprises a quarterwave plate.
 12. A method as claimed in claim 11 wherein a meanderline isused as the quarter wave plate.
 13. A method as claimed in claim 10wherein the polarisation dependent reflector comprises a plurality ofparallel conducting tracks.
 14. A method as claimed in claim 10 whereinthe polarisation dependent reflector is arranged to reflect radiationcoming from a single general direction.
 15. A method as claimed in claim10 wherein the polarisation dependent reflector is divided into two ormore parts, each arranged to reflect radiation coming from a differentgeneral direction towards the scene.
 16. (canceled)
 17. A method ofilluminating a target area using an off-axis illumination sourcecomprising the steps of: a) providing an illuminator generatingradiation at a desired wavelength in the millimetre waveband orthereabouts; b) reflecting radiation from the illuminator having a firstpolarisation state and passing radiation having a second polarisationstate, by means of a polarisation dependent reflector; c) processing thereflected radiation using an optical component, whereby the opticalcomponent changes the polarisation state of the radiation, and passingthis processed radiation towards a target area; d) passing at least someof the radiation reflected from the target area back towards the opticalcomponent; and e) changing the polarisation of the radiation reflectedfrom the target area to the second polarisation state such that it canpass through the polarisation dependent reflector (10) and on to amillimetre wave imager.
 18. A method as claimed in claim 17 wherein theoptical component comprises a quarter wave plate.
 19. A method asclaimed in claim 18 wherein a meanderline is used as the quarter waveplate.
 20. A method as claimed in claim 17 wherein the polarisationdependent reflector comprises a plurality of parallel conducting tracks.21. A method as claimed in claim 17 wherein the polarisation dependentreflector is arranged to reflect radiation coming from a single generaldirection.
 22. A method as claimed in claim 17 wherein the polarisationdependent reflector is divided into two or more parts, each arranged toreflect radiation coming from a different general direction towards thescene.
 23. (canceled)